Urethane resin particles, which have excellent physical properties, have been widely used as resin particles for use in coating compositions, for example, to improve coating film performance.
Acrylic urethane composite resin particles have also been used in the fields such as coating compositions, for example, to obtain the synergistic effects of acryl and urethane, improve compatibility when applied to acrylic resin-based coating compositions, and reduce the cost of urethane resin particles.
Based on the manner of dispersion in aqueous systems, synthesis of acrylic urethane composite resin particles can be roughly categorized into (i) self-emulsifying particles and (ii) particles dispersed using an emulsifier.
As an example of the self-emulsifying particles, Patent Literature 1 discloses a method for producing an aqueous dispersion of a urethane-acrylic inner particle mixture with excellent film-forming properties and water resistance, wherein seed polymerization of an acrylic monomer is performed using, as a seed, an aqueous dispersion of urethane that can be prepared without using an emulsifier, and seed polymerization of an acrylic monomer is then performed again using the obtained aqueous dispersion of an inner particle mixture of urethane and acryl as a seed. Patent Literature 1 discloses that the aqueous dispersion of a urethane-acrylic inner particle mixture can be used for film-forming materials, coating compositions, adhesives, or pressure sensitive adhesives.
However, when such self-emulsifying acrylic urethane composite resin particles are used for an aqueous coating composition, the compatibility with other resin components is insufficient, and the obtained coating film may be cloudy, resulting in unsatisfactory finished appearance. Further, there may be a problem with the storage stability of the aqueous coating composition because of insufficient dispersion stability.
As an example of the particles dispersed using an emulsifier, Patent Literature 2 discloses a method for producing an aqueous dispersion of a (meth)acrylic composite resin by sequentially or continuously adding starting-material emulsions to an aqueous medium in which the emulsions have one or more (meth)acrylic monomers dispersed beforehand, to perform emulsion polymerization of the one or more (meth)acrylic monomers in the presence of a urethane resin. Patent Literature 2 discloses a fiber composite, fibrous base material laminate, and the like obtained using the aqueous dispersion of a (meth)acrylic composite resin.
However, likewise, when such acrylic urethane composite resin particles dispersed using an emulsifier are used for an aqueous coating composition, the compatibility with other resin components is insufficient, and the obtained coating film may be cloudy, resulting in unsatisfactory finished appearance. Further, since such acrylic urethane composite resin particles require a substantial amount of an emulsifier, there may be a problem with the resistance to water load (e.g., water resistance) of the obtained coating film when the particles are used for a coating composition.
Patent Literature 3 discloses an aqueous dispersion of a polyurethane-(meth)acrylic polymer composite resin obtained by dispersing polyurethane and a (meth)acrylic polymer in an aqueous medium, wherein constitutional units derived from the diols constituting the polyurethane and/or constitutional units derived from the polyvalent isocyanates constituting the polyurethane contain a cyclohexane ring, the cyclohexane ring content is 2.5 mol or more per kg of polyurethane, and the (meth)acrylic polymer has a glass transition temperature (Tg) of 50 to 160° C. Patent Literature 3 discloses that an aqueous coating agent and the like containing the composite resin dispersion are excellent in, for example, adhesion to resin substrates such as ABS and metal substrates such as aluminum.
However, likewise, when such an aqueous dispersion of a polyurethane-(meth)acrylic polymer composite resin is used for an aqueous coating composition, since the compatibility of the aqueous dispersion of a polyurethane-(meth)acrylic polymer composite resin with other resin components is insufficient, the obtained coating film may have unsatisfactory finished appearance. Further, there may be a problem with the storage stability of the aqueous coating composition because of insufficient dispersion stability of the aqueous dispersion of a polyurethane-(meth)acrylic polymer composite resin.